GB2083325A - Display system - Google Patents

Abstract

A compressed data store 2 holds data (especially graphics) in an encoded format for decompression 25 and display on a raster- scanned C.R.T. monitor. The encoded data consists of a string of codewords divided into sections, each corresponding to a zone of the monitor and each section ending in an end-of-zone codeword. The codewords of each section are stored so as to be retrieved in sequence in response to addresses supplied from a counter 46. In response to an end-of-zone codeword the counter 46 is reset to the address of the first codeword of the next section, which is supplied from a RAM 48 accessed sequentially by a counter 49, allowing sections of codewords to be stored out of sequence and non-contiguously, which facilitates replacement of sections to change part only of the display. The new data is written into the store 2 from an external processor, which then inserts the appropriate starting address(es) in the RAM 48. <IMAGE>

Description

SPECIFICATION Improvements in or relating to display systems This invention relates to display systems, and in particular to display systems in which a display is produced on a display device by repeatedly scanning a plurality of picture elements in a predetermined sequence. The most common current example of a display device operating in this manner is the raster-scanned refreshed cathode-ray tube.

If the picture elements are to be scanned repeatedly information defining the display value (that is colour and/or intensity) of each picture element must be stored and then supplied once per scan. In the case of systems designed to display graphics information it has been proposed to store the information in a form in which a separate item of information is present for each picture element of the display. This arrangement requires a large storage capacity, especially if a reasonable range of possible display values is to be allowed for each picture element of the display. It has therefore been proposed to store the information in a compressed form related to the structure of the information to be displayed.The compressed representation of the display is then output once for each occasion the display is scanned and decompressed to produce a separate item of information for each picture element.

One characteristic of a compressed representation is that it is liable to be of variable length. In general, the greater the regularity of the display, the greater the degree of compression. That makes it difficult, if it is desired to change a part only of the display, to change the corresponding part of the representation, since the new part is liable to be of a different length to the original part.

This invention provides apparatus for producing a drive output for supply to a display device to control the display value of each picture element of a display formed of a plurality of picture elements repeatedly scanned in a predetermined sequence, the apparatus comprising: compressed-data storage means for storing a compressed representation of the display, which representation is divided into a plurality of sections, each section consisting of a sequence of code elements defining the display value of each picture element in a sequence of consecutively scanned picture elements and the end of each section being indicated by a code element of a distinctive kind;; access means capable in normal operation of retrieving code elements from the compresseddata storage means and outputting them in a predetermined sequence determined by their storage positions, and also settable in accordance with a supplied indication to retrieve and output a code element held in a position in the compressed-data storage means defined by the indication; the code elements of each of the said sections being stored in the compressed-data storage means in positions such that the access means in normal operation will retrieve the code elements of each section and output those code elements in the order in which they occur in the section;; resetting means arranged in response to the detection of a code element of the said distinctive kind output from the compressed data storage means to supply an indication of a starting point to the access means and to reset them in accordance therewith, the starting point indicated defining, in operation, the storage position of the first code element of the section of the compressed representation next to be output after that containing the code element of the distinctive kind concerned; and decompressing means connected to receive code elements output from the compresseddata storage means and arranged repeatedly to decompress the compressed representation to produce a drive output for supply to a display device to control the display value of each picture element as it is scanned in the said predetermined sequence.

The various sections of the representation are therefore retrieved in the correct sequence even if they are not stored consecutively. If it is desired to change a part only of the display one or more new sections may be introduced into the compressed-data storage means and their starting-point indications supplied to the access means in place of those of the sections they are to replace. The new section cr sections may be stored in a different part of the compressed-data storage means and need not be the same length as the section or sections replaced.

Preferably the resetting means includes means for storing a plurality of starting-point indications, access means for the resetting means arranged repeatedly to retrieve and output the starting-point indications in a predetermined order and means arranged in response to the detection of a code element of the said distinctive kind to cause the last-mentioned access means to retrieve and output the next required starting-point indication for supply to the access means for the compressed-data storage means for resetting those means.

Preferably the compressed representation comprises code elements each of which represents a picture element or a plurality of consecutively scanned picture elements, consecutive such code elements representing consecutively scanned picture elements.

The invention also provides a display system comprising apparatus according to the invention and a display device arranged to receive the said drive output and in response thereto to produce the said display.

The invention further provides a method of driving a display device to cause it to produce a display formed of a plurality of picture elements scanned in a predetermined sequence, the method comprising: storing a compressed representation of the display in compressed-data storage means, the compressed representation being divided into a plurality of sections each consisting of a sequence of code elements defining the display value of each element in a sequence of consecutively scanned picture elements and the end of each section being indicated by a code element of a distinctive kind;; repeatedly outputting the compressed representation from the compressed data storage means, the compressed representation being output on each occasion section by section and each section being output by supplying a starting-point indication for that section defining the storage position of the first code element of the section, and outputting the code elements of the section in order, starting with the said first code element, the code element of the said distinctive type indicating the end of the section being detected when output and the startingpoint indication of the section of the compressed representation next to be output being supplied in response thereto to define the storage position of the first code element of that section for its output;; and decompressing the compressed representation each time it is output to provide a drive output supplied to the display device which output controls the display value of each picture element of the display as it is repeatedly scanned in the predetermined sequence.

The display produced in accordance with the invention may advantageously be changed from its original to a new form by, while the original display is being produced, entering into a part of the compressed-data storage means not used to store the compressed representation of the original display, a replacement section or replacement sections for the compressed representation, which section defines or sections define picture elements of the display that comprise those to be changed, the or each replacement section being a section as specified above, and thereafter, to control the output of data from the compressed-data storage means, supplying the starting point indications of the replacement section (s) in the place of the starting-point indication(s) of the section(s) of the original compressed representation defining the same picture elements of the display as the replacement section(s) so as to cause the compressed-data storage means to output a new compressed representation containing the replacement section(s).

Apparatus constructed and operating in accordance with the invention will now be described in greater detail by way of example with reference to the accompanying drawings, in which Figure 1 is a block diagram of the overall display system; Figure 2 is a diagrammatic representation of a display; Figure 3 is an illustration of the use of run-length and transition code words, Figure 4 is an illustration of the use of a correlation code word, Figure 5 is a block diagram of the decompressor of Fig. 1; Figure 6 is a block diagram of part of the control unit of Fig. 5; Figure 7 is a diagram of the way the display may be divided into zones; Figure 8a is a diagram illustrating the way the compressed representation of the display may be arranged in the compressed data store of Fig. 1;; Figure 8b shows the organisation of the compressed data store while the compressed representation of Fig. 8a is being changed; Figure 8c shows the final organisation of the compressed data store after the change illustrated in Fig. 8b has taken place; and Figure 9 shows the addressing circuit of Fig. li.

Outline of the system Referring to Fig. 1, the display system forms its display on a display device consisting of a standard TV colour monitor 1, which produces the display by a raster scan of two interlaced fields. The information to be displayed is held in a compressed form in a compressed data store 2 and output once for each complete scan of the display. It is received by a decompressor 3 which decompresses it and provides the signals to drive the monitor 1. The compressed representation of the display that is held in the store 2 consists of a sequence of codewords and each in turn is output in the correct sequence when called for by the decompressor 3 on control lines 4. The codewords are not required to be stored in sequentially addressable locations in the store 2; the correct location is accessed by an addressing circuit 5 to which the control lines 4 are connected.

The compressed data is supplied over lines 6 from an external processor (not shown). The lines 6 can load data in both the store 2 and the addressing circuit 5.

Coding of the display The system is designed primarily to display graphics data such as lines, figures or diagrams, although of course text may also be included. A typical display is shown in a highly stylised form in Fig. 2. It is formed of an array picture elements 10 arranged along the scan lines 11 traced by the beam of the monitor 1. The visual appearance of each picture element may be separately controlled by brighting up the gun or guns of the monitor to the required extent at the appropriate point in the scan.

In Fig. 2 the display shows a quadrilateral made up of picture elements 1 Oa of one display value (that is, colour or grey scale) contrasted with a background made up of picture elements 1 Ob of another display value.

In the system to be described, the display value of each picture element 10 is defined by three bits of information, defining the on/off state of the red, blue and green guns. That gives eight display values in all, but of course if a wider range of colour and/or brightness is desired,more bits may be provided so that the intensity of each gun can be controlled as well.

The codewords of the compressed representation, in the order they occur in the representation, define the display values of the picture elements in the order they are scanned. The main class of codewords each represent a group of consecutively scanned picture elements and indicate the display value of each element either by information in the codeword or by reference to the display value of a previously scanned element. The picture elements represented by any codeword are not defined in the codeword itself, but are determined by the preceding part of the compressed representation.

In the usual case, where the codeword immediately follows another such codeword, the picture elements represented are those which follow, in the order of the scan, those represented by the immediately preceding codeword.

There are three types of codeword which represent picture elements in the manner described.

They will be termed transition, run-length and correlation codewords.

Transition codewords each represent a pair of picture elements and indicate the display value or values of the pair.

Run-length codewords each represent a segment of picture elements of the same display value and indicate its length but not its display value. Provided the segment does not start a scan-line, the display value of the elements of the segment is the same as that of the immediately preceding element. If the run-length code does start a scan-line it indicates not only the length of the segment but also that the segment has a predetermined display value, chosen here to be black, since that is the commonest background for a display.

Any scan line can be represented by a sequence of transition and run-lengths codewords.

Runs of elements of the same colour are divided so that the first element of the run is indicated by a transition codeword, and an immediately following run-length codeword indicates the length of a segment which containues the run. If necessary one or more further run-length codewords may follow and the run may be concluded by another transition code. This scheme allows runs of any length to be encoded, whether of odd or even length, even though the codewords divide the scane line up in groups containing even numbers of elements. This is illustrated by Fig. 3, in which the start of a scan-line is shown in which a run of six red picture elements is followed by a run of seven green elements and the start of a run of blue elements, the elements of the three colours being denoted by R, G and B respectively.The elements represented by the same codeword are shown bracketed, and the type of codeword is shown as T for a transition and RL for a run-length.

The third type of codeword so far mentioned, the correlation code, is designed to take advantage of the fact that transitions from one display value to another often lie on lines forming the borders of objects, in which case the position of the transition in one scan-line is correlated with that in previous scan-lines. A correlation codeword can represent all the elements up to and including the first to follow the next transition to a new display value, and indicates the position of the transition relative to its position in the preceding line scanned in that field, that is, whether it is vertically beneath it or an indicated number of positions to left or right. If consecutive transitions are all related in position to those in the earlier line in the same way, they too may be included in the codeword.Fig. 4 shows the elements that would be represented by a correlation codeword indicating that two transitions each occur two positions to the left of their positions in the previous line scanned in that field.

Besides the representational codewords described above, there are control codewords connected with the end of scan-lines.

The codewords are eight-bit binary numbers, but expressing them as octal numbers for convenience, the complete repertoire of codewords is as follows: Codeword Type Significance 001 to 200 run length length of run divided by two 201 control end of field 202 control end of line 203 control end of frame 206 control end of zone 210 to 277 correlation second digit type of correlation: 1 6 elements right 2 4 elements right 3 2 elements right 4 vertical 5 2 elements left 6 4 elements left 7 6 elements left third digit the number of correlations (0 = 8 correlations) 300 to 377 transition the second and third digits represent the colours of first and second elements respectively, as follows:: 0 black 1 red 2 green 3 yellow 4 blue 5 magenta 6 cyan 7 white With some differences in the control codewords this coding scheme is the same as that described in our British Patent Specification 14122/77 and the method of encoding the display is described there in greater detail.

Decompressor Referring to Fig. 5, the decompressor 3 has a control unit 1 5 to which the individual codewords are supplied. They are each decoded, and the appropriate bits of colour information for each picture element are supplied to the monitor 1 as a drive output on lines 1 6a-1 6c in synchronism with the scan of the screen. The scan is controlled by a video sync waveform supplied by a sync unit 17, which also supplies timing signals for other parts of the decompressor 3, including clock signals at the picture element rate and half that rate (that is, once every two picture elements).

In response to a transition codeword the control unit 1 5 supplies a START signal to a transition logic unit 18, together with the two display values concerned. These values are clocked out from there in turn at the appropriate times and supplied to the monitor on the lines 1 6 a to 16 c. The last of the two display values is preserved in the transition unit 1 8 and a FINISHED signal is returned to the control unit 1 5.

In response to a run-length codeword the control unit 1 5 sends a START signal to a runlength logic unit 19, together with an indication of the length of the run. The run-length logic unit 1 9 is basically a counter which is counted down once every two picture elements while the display value held at that time in the transition logic unit 1 8 is output at the picture-element rate to control each corresponding picture element of the display. When the counter reaches zero a FINISHED signal is returned to the control unit 1 5.

To allow correlation codewords to be used, the display-value information output from the transition logic unit 1 8 is also stored in a line store 20, which is functionally a recirculating shift register, although it is advantageously implemented as two random-access buffers addressed by suitable number. The division of the display into zones is relevant to the way the compressed data is stored, but is not intended to be visible to the user and the boundaries between the zones are imaginary.

The compressed data store 2 has separately addressable locations each of which is able to hold a codeword. The compressed representation is divided into sections each consisting of the part of the representation representing the scan lines of one field in a particular zone. The codewords of each section, in the order they occur in the section, are stored in the compressed data store in locations the addresses of which are in an ascending sequence. But the different sections are not necessarily contiguous nor stored in the correct order.

One example of the way the data may be organised in the compressed data store 2 is shown diagrammatically in Fig. 8a. In this figure, the locations of the store 2 are considered to be arranged vertically in the order of their addresses, with the lowest at the top, and for simplicity it is assumed for the present that the display is non-interlaced, so each section holds all the codewords for one zone.

As shown, sections 37 to 40 contain the data for the zones 1 to 4 respectively. In this instance they are in the correct order, but are not all contiguous.

Each section starts with the codeword defining its first picture element, and the sections 37 to 39 representing the first three zones end with an end-of-zone code 41. The section 40 representing the final zone ends with an end-of-frame codeword 42. The address of the location holding the first codeword in each of the section 37 to 40 is shown as Al to A4 respectively.

Addressing circuit and compressed data store Referring to Fig. 9, the compressed data store 2, which holds its data in the manner just described, has an output register 45 which is enabled to supply a codeword on the lines 25 to the decompressor on receipt of the NEXT signal. The store 2 is addressed by a counter 46, which is incremented in response to the NEXT signal and is reloaded from a register 47 in response to the EOZ signal indicating the end of a zone or field. The register 47 is the output register for a random-access memory 48 the address for which is normally supplied by a counter 49 which is incremented in response to the end-of-zone signal EOZ and cleared in response to the end-of-frame signal EOF.

The store 2 is dual-ported to allow the external processor to write data to it or read data from it.

The counter 49 supplies its output to the RAM 48 through a multiplexer 50 which also has an input connected to the address but of the external processor. The locations of the RAM 48 are assigned values in the processor's address space and when a decoder 51 recognises that the address carried by the address bus refers to one of these locations it selects the input to the multiplexer 50 connected to the processor's address bus, thereby giving the processor access to the RAM 48 to write to it or read data from it.

The RAM 48 contains the addresses Al, .2... etc. of the initial codeword in each section in the correct order in consecutive locations starting from location 0 of the RAM 48.

The operation of the addressing circuit 5 will now be described, starting at a point where the scan is in the middle of a zone which is not the final one. When the decoding of each codeword has been completed the decompressor 3 sends the NEXT signal to the addressing circuit 5 and the next codeword is supplied from the register 45. The counter 46 is then incremented and the contents of the corresponding location read into the register 45. Since within the section the codewords are arranged in locations with consecutive addresses this codeword is the next to be required by the decompressor 3.

When the end of the zone is detected the decompressor sends the EOZ signal to the addressing circuitry. This causes the counter 46 to be loaded with the address held in the register 47, which is arranged to be that holding the starting address for the next section, that is, the address of the location holding its first codeword. This codeword is then read into the register 45, which is enabled to supply the codeword to the decompressor so that it undergoes its preliminary decoding by the PROM 26, so as to be ready to initiate the required logic unit when the line flyback signal causes NEXT to be again issued to start the next line.

The EOZ signal also causes the counter 49 to be incremented and the contents of the RAM 48 at the corresponding address to be read into the register 47 ready for the next EOZ signal.

The register 47 therefore now holds the starting address of the next section.

When the decompressor detects the end of a frame it issues the EOF signal, which causes the counter 49 to be cleared and the contents of the corresponding location of the RAM 48-location 0, which holds the starting address of the first section of the compressed representation-to be read out. The corresponding codeword is then passed to the decompressor 3, and the scan of the picture elements starts again.

Control signals to cause the address circuit 5 to operate as described are derived from the NEXT, EOZ and EOF signals, with the introduction of suitable delays.

It may be convenient to implement the store 1 in a form in which several codewords are read counters, one accepting the current scan line and the other outputting the previous one. At the end of each scan line the roles are reversed.

In response to a correlation codeword the control unit issues a START signal to a correlation logic unit 21, together with information defining the type and number of correlations. In the correlation logic unit 21 the display values output from the line store 20 pass through a shift register from which the value of the picture element of the previous line bearing the appropriate timing relationship with the current picture element is selected and output from the decompressor. Meanwhile the output of the transition logic unit 1 8 is inhibited. Transitions are detected by comparing the display value of the current picture element with that of the preceding one and after the appropriate number have been detected, as indicated by a counter, the FINISHED signal is returned.

Suitable logic units 18, 19 and 21 are described in greater detail in our above-mentioned patent specification. The system described herein uses the modification described in the earlier specification according to which display values are output from the transition logic unit one at a time. The alternative described in the earlier specification according to which they are output in pairs could also be used.

Referring to Fig. 6, the control unit 1 5 receives the incoming codeword from the compressed data store 2 on lines 25. The codeword addresses a programmable read-only memory 26, the appropriate location of which contains a three-bit indication of the type of the codeword concerned. This indication is supplied to an eight-to-one decoder 27 and also latched in a register 28. When the decoding of the previous codeword has been completed the decoder 27 is enabled by a signal on a line 29 and the decoder 27 issues a signal on one of its output lines.

If the codeword supplied was a transition, run-length or correlation codeword the signal is the START signal to the corresponding logic unit 18, 19 or 21. The information defining the details of the codeword concerned is taken directly from those of the lines 25 carrying the appropriate bits for supply to the logic unit 18, 19 or 21.

The FINISHED signal from each of these units is supplied to a multiplexer 30, the select input of which is the output of the register 28. The select input ensures that only the FINISHED signal from the expected unit (defined by the contents of the register 28) is passed through the multiplexer 30. The output of the multiplexer 30 forms both the enabling signal on the line 29 which initiates the decoding of the next codeword at the individual picture-element level, and also the NEXT signal which causes the retrieval of the codeword following that one.

So far, the way the control unit 1 5 deals with run-length, transition and correlation codewords has been described. However, at the end of a scan line one of the control codewords must be included. The codewords for most scan lines end with an end-of-line codeword. But the final scan-lines of the first and second fields end in an end-of-field codeword and an end-of-frame codeword respectively. And some other scan lines, as will be explained, end in an end-of-zone codeword.

An end-of-line codeword is decoded by the PROM 26 to give an output which is latched in the register 28 but has no corresponding output from the decoder 27. Therefore when the decoding of the preceding codeword has been finished the next codeword will be called for by the NEXT signal, but no action will be initiated by the decoder 27. The display value of the last picture element of the preceding codeword will then continue to be clocked out of the transition unit to control the display value of any succeeding picture elements in the scan-line concerned.

The end-of-line codeword may therefore be used in place of a final run-length codeword to represent the final segment of the scan line.

The unit of the multiplexer 30 selected by the value held in the register 28 is connected to receive the line flyback signal from the sync unit 1 7. When it is received it is passed on to initiate the generation of the display values of the initial picture elements of the net line to be scanned.

The other control codes act similarly as far as the decompressor is concerned, selecting an input to the multiplexer 30 connected to receive the appropriate timing signal, but they also cause outputs from the decoder 27 which are supplied to the addressing circuit 5. Thus the end-of-frame codeword causes the output of a signal EOF on a line 31, and the end-of-zone and end-of-field codewords both cause the output of a signal EOZ on a line 32.

Division of the display into zones The length of the compressed representation is variable, depending on the display to be represented. This makes it difficult, if part only of a display needs to be changed, to change the appropriate part of the representation. Even if the original part can be located, the new part is likely not to be of the same length as that it is to replace.

To overcome this problem the display is regarded as divided into zones, as illustrated in Fig.

7, where a display 35 is shown divided into four zones 36. This number is chosen for convenience illustration and smaller than is likely to be used in practice: 1 6 has proved a out in parallel into registers which are then enabled in turn to obtain the individual codewords.

With such an arrangement each starting address held in the RAM 48 defines a group of locations, and the initial codeword of each section can only be placed in a location which is the first of such a group, in order to ensure that it is retrieved when required.

The use of interlace affects the organisation of the store 2 to this extent, that the final section of the first field ends in an end-of-field, not an end-of-zone codeword, and that there will be two sections for each zone, representing the parts of the two fields falling in that zone. In the addressing circuit 5 the RAM 48 holds the starting addresses for the sections of the first held, followed consecutively by those of the second field. But its operation is not affected: both endof-zone and end-of-field codewords produce an EOZ signal and the first section of the second field follows the last of the first field automatically.

Changing the display One or more zones of the display may be changed as follows. First, the processor writes the required new sections of the compressed representation into areas of the store 1 which it knows to be free from a record it keeps of start and end locations of sections. Fig. 8b shows a new section 55 with a starting address A3' being introduced to change the data displayed in zone 3.

The processor then writes the appropriate starting addresses into the RAM 48 in place of those of the sections to be replaced-in the example of Fig. 8b the address A3' replaces A3 in the third location. The display produced will then include the new zone 3 in place of the original one, and the organisation of the store 1 will appear as shown in Fig. 8c. The sections are now out of order.

It is perfectly possible, provided there is enough free space, for the new section to be longer than the one it replaces.

The use of the RAM 48 in conjunction with end-or-zone/end-of-field codewords allows the display to be changed in a particularly easy manner. And it provides considerable flexibility. For example two separate compressed representations may be held in the store 1, and the display switched from one to the other, without the need to rewrite the compressed data store, by changing the addresses in the RAM 48. If the compressed representation for a full interlaced display is too long in any particular case to fit into the store 1, vertical resolution may be reduced by using a compressed representation detailing one field only and ending with an endof-frame codeword. Both fields, as displayed, will then be identical.

General Correlation codewords are not used in coding the first scan-line of any zone in either field. If they were, that line would be tied to the final line in the corresponding field of the preceding zone and zones could not be changed independently.

Text may be included by treating it as any other graphics data and including it in the compressed representation for decompressing by the compressor. As an alternative, a separate page store and character generator of the teletext type may be included, the page store being written to by the processor and the output of the character generator being combined with that of the decompressor for supply to the monitor.

The compressed representation may indicate display values indirectly, rather than, as has been so far described, directly. Thus, the output on the lines 1 spa to 1 sic, instead of defining the actual colour and/or grey scale of the corresponding picture element and being supplied to the monitor, may act as an addressing signal for a look-up table, or "colour map", held in randomaccess storage, the output of which is supplied to the monitor. This storage may be wntten to by the external processor. With this arrangement, although the maximum number of display values that can be used at any one time is eight (for a coding scheme with, as here, three bits of display-value information per picture element) the total number of display values can be much greater and depends on the length of the word stored in the random-access storage. The processor can then change the display values used by writing new values into this storage.

The invention is equally applicable to other coding schemes, for example one in which runs are represented by codewords each indicating both the display-value and the length of a run, rather than one which, as described, divides runs and uses both transition and run-length codes.

Claims (8)

1. Apparatus for producing a drive output for supply to a display device control the display value of each picture element of a display formed of a plurality of picture elements repeatedly scanned in a predetermined sequence, the apparatus comprising: compressed-data storage means for storing a compressed representation of the display, which representation is divided into a plurality of sections, each section consisting of a sequence of code elements defining the display value of each picture element in a sequence of consecutively scanned picture elements and the end of each section being indicated by a code element of a distinctive kind;; access means capable in normal operation of retrieving code elements from the compresseddata storage means and out-putting them in a predetermined sequence determined by their storage positions, and also settable in accordance with a supplied indication to retrieve and output a code element held in a position in the compressed-data storage means defined by the indication; the code elements of each of the said sections being stored in the compressed data storage means in positions such that the access means in normal operation will retrieve the code elements of each section and output those code elements in the order in which they occur in the section;; resetting means arranged in response to the detection of a code element of the said distinctive kind output from the compressed data storage means to supply an indication of a starting point to the access means and to reset them in accordance therewith, the starting point indicated defining, in operation, the storage position of the first code element of the section of the compressed representation next to be output after that containing the code element of the distinctive kind concerned; and decompressing means connected to receive code elements output from the compresseddata storage means and arranged repeatedly to decompress the compressed representation to produce a drive output for supply to a display device to control the display value of each picture element as it is scanned in the said predetermined sequence.

2. Apparatus as claimed in claim 1, in which the resetting means includes means for storing a plurality of starting-point indications, access means for the resetting means arranged repeatedly to retrieve and output the starting-point indications in a predetermined order and means arranged in response to the detection of a code element of the said distinctive kind to cause the last-mentioned access means to retrieve and output the next required starting-point indication for supply to the access means for the compressed-data storage means for resetting those means.

3. Apparatus as claimed in claim 1 or claim 2, in which the compressed representation comprises code elements each of which represents a picture element or a plurality of consecutively scanned picture elements, pairs of such code elements occurring consecutively in the representation representing consecutively scanned picture elements.

4. A display system comprising apparatus as claimed in any one of the preceding claims and a display device arranged to receive the said drive output and in response thereto to produce the said display.

5. A display system substantially as hereinbefore described with reference to the accompanying drawings and as shown in Figs. 1, 5, 6 and 9 of those drawings.

6. A method of driving a display device to cause it to produce a display formed of a plurality of picture elements scanned in a predetermined sequence, the method comprising; storing a compressed representation of the display in compressed-data storage means, the compressed representation being divided into a plurality of sections each consisting of a sequence of code elements defining the display value of each element in a sequence of consecutively scanned picture elements and the end of each section being indicated by a code element of a distinctive kind;; repeatedly outputting the compressed representation from the compressed data storage means, the compressed representation being output on each occasion section by section and each section being output by supplying a starting-point indication for that section defining the storage position of the first code element of the section, and outputting the code elements of the section in order, starting with the said first code element, the code element of the said distinctive type indicating the end of the section being detected when output and the startingpoint indication of the section of the compressed representation next to be output being supplied in response thereto to define the storage position of the first code element of that section for its output;; and decompressing the compressed representation each time it is output to provide a drive output supplied to the display device which output controls the display value of each of the display as it is repeatedly scanned in the predetermined sequence.

7. A method as claimed in claim 6, in which the compressed representation comprises code elements each of which represents a picture element or a plurality of consecutively scanned picture elements, pairs of such code elements occurring consecutively in the representation representing consecutively scanned picture elements.

8. A method as claimed in claim 6 or claim 7, in which the display is changed from its original to a new form by, while the original display is being produced, entering into a part of the compressed-data storage means not used to store the compressed representation of the original display a replacement section or replacement sections for the compressed representation, which section defines or sections define picture elements of the display that comprise those to be changed, the or each replacement section being a section as specified in claim 6, and thereafter, to control the output of data from the compressed-data storage means, supplying the starting point indications of the replacement section(s) in the place of the starting-point indication(s) of the section(s) of the original compressed representation defining the same picture elements of the display as the replacement section(s) so as to cause the compressed-data storage means to output a new compressed representation containing the replacement section(s).